Unlike Reaper that is just going to wreck you, this is more of a long battle, what I was thinking is you have a Titan style monster, that has a crap ton of Hp no immunities but no weakness, he deals melee damage, then at every 25% his health has been reduced he does an AOE ranged attack that hits everyone in the rooms and spawns a smaller versions of himself = to number of players in the room,(Scaled, easy 75% remaining health, medium 50%, hard 25%.) His smaller forms will drop drops for the people ranging outside out tanking, and then the tank will get the bigger drop but, not the 3 little ones, everyone would get xp, either from party system or damage done. Scaled mini versions of him would hold immunities or protections from range, mage, and melee. Not in any given order. But so everyone would have to change "classes" at some point. You could still solo it, it would just be implausible.